This application relates to acoustic deterrence and, in particular, to an acoustic deterrent device and methods for deterring mammals, most particularly but not exclusively marine mammals.
Worldwide, farming of marine and diadromous finfish species has experienced tremendous growth rates, showing a ten fold increase over the last three decades. This increase in potential food resources presented in a marine environment has brought about increased interactions with predatory species. One common group of predators is marine mammals who exploit food resources depending on their profitability and potential costs, which include dive depths as a major factor. The shallow depth of fish farms thus makes them particularly attractive to predators.
In particular, predatory behaviour of pinnipeds is a major concern, causing a variety of economical and market related risks for the fish farm owner. Accordingly, there is much interest in developing anti-predator control methods.
These methods include net modifications, lethal or non-lethal removals, population control and aversive conditioning. However, each of these methods has their own drawbacks. For example, the addition of a second net can cause tangling of predators and non-predatory species; and lethal removals as well as population control may have an impact on populations and raise ethical concerns over the treatment of the animals. Culling of higher order predators can also have negative impact on predation rates by other predators, for example, pinnipeds forage on predatory fish species around the net pen which in turn potentially feed on aquaculturally important species. Also, emetic aversion conditioning requires that individuals learn to associate treated fish with sickness, and this can be hard to achieve when predator numbers are high.
One anti-predator control method that avoids these pitfalls is the use of acoustic deterrent devices (ADDs). These have traditionally been considered to be a benign solution. However, they do present certain problems with respect to the effects they have on other marine wildlife and with habituation, where a target species motivated by a food source ceases to be deterred by the acoustic signals.
If the source level of an ADD is sufficiently high it can cause temporary or permanent hearing damage both to the targeted species and to other wildlife, and the noise pollution is in general an environmental hazard. Further, both targeted and non-targeted species can be excluded from their natural habitat within a wide radius of the fish farms. These concerns have led some governments to restrict or even ban the use of ADDs.
A further problem is the habituation of the target species to the sound. In extreme cases, the sound which is intended to be aversive acts as a “dinner bell” and actually serves to attract predators, rather than deter them. Also, if predator sounds are used as an aversive noise, habituation is dangerous for the target species once they had returned to their normal habitat.
Also, existing power levels and signal cycles impose heavy duty cycles on the batteries used as power sources in the transducer units.
Accordingly, it would be desirable to provide an acoustic deterrent device that is highly effective, but does not damage the environment, is species specific and avoids habituation.
According to a first aspect of the present invention there is provided a method of deterring mammals comprising configuring an acoustic deterrent system to emit an acoustic signal having characteristics which repel the mammals by eliciting an acoustic startle response reflex in said mammals.
“Deterring” is taken to mean discouraging or preventing a mammal from entering into or staying in a particular area. The startle reflex response is a physiological reflex in mammals to particular sounds, which often initiates flight from the sound, thereby repelling mammals from a location in the vicinity of the sound. The startle reflex response should not be confused with an animal or person being “started” or “startled” in the colloquial or everyday sense.
In an embodiment, the method of incurring an acoustic startle reflex response comprises the steps of:                selecting a target order, family, genus or species of mammals to be deterred;        selecting a received level at a predetermined level above a representative hearing threshold of the targeted mammals;        transmitting an acoustic signal from a transmission point at a source level required, taking into account transmission loss, to create the selected received level at a predetermined distance from the transmission point.        
In an embodiment, the predetermined level is between 90 dB and 125 dB re 1 μPa above the representative hearing threshold at each frequency.
In an embodiment, the acoustic signal has a duration about as long as the acoustic integration time specific to the targeted mammal's auditory system
In an embodiment, the acoustic signal has a duration of less than 200 ms.
In an embodiment the acoustic signal has a rise time of less than 20 ms.
In an embodiment, the acoustic signal is as broadband as possible within the designated frequency range.
In an embodiment, the acoustic signal comprises frequency components at which the aural sensitivity of the targeted mammals is greater than that of selected other animals.
In an embodiment, the acoustic signal uses a frequency band between 500 Hz and 2 kHz for the deterrence of seals or other pinnipeds. The acoustic signal is as broadband as possible within this frequency range.
In an embodiment, the mammals to be deterred comprise sea mammals including pinnipeds, such as seals; or cetaceans.
Alternatively, the mammals to be deterred comprise land mammals.
In an embodiment, a secondary conditioning sound is made to condition the mammals to avoid the main stimulus. This could be a sound of centre frequency 1 kHz, modulation rate 250 Hz and duration 1.2 seconds with two modulation cycles.
In an embodiment, the conditioning sound is played between 500 ms and 5 s before the main acoustic signals on selected occasions.
According to a second aspect of the invention, there is provided a method of deterring marine mammals comprising transmitting an acoustic signal which comprises an aversive sound whose characteristics are chosen based on characteristics that are unpleasant to humans.
In an embodiment, the aversive sound is selected to have one or more of the following psychophysical features: high roughness; low tonality; high loudness; high sharpness.
In an embodiment a frequency modulation is applied to the carrier signal, most preferably with a modulation frequency of about 70 Hz.
This frequency modulation is applied in order to achieve high roughness.
In an embodiment, the frequency modulation has a modulation depth of between 10 and 150%, preferably 50% of the centre frequency of the carrier signal.
In an embodiment, the aversive sound has a sound pressure level of at least 70 dB above the hearing threshold of the targeted mammals, most in an embodiment at least 80 dB above the hearing threshold of the targeted mammals.
In an embodiment, an aversive sound is of a complex form and comprises partials that fall within 25% of the critical bandwidth of the hearing system of the targeted mammals.
In an embodiment, when cetaceans are to be deterred the acoustic signals could be close to the upper frequency edge of the hearing range in order to increase sharpness.
In an embodiment, the acoustic signal is as broadband as possible within the selected frequency band in order to increase loudness without increasing the actual sound pressure level.
In an embodiment, the aversive sound comprises frequency components at which the aural sensitivity of the targeted mammals is greater than that of selected other animals.
In an embodiment, the acoustic signal comprises one or more frequency components between 500 Hz and 2 kHz for the deterrence of seals or other pinnipeds.
In an embodiment, sound exposure time is determined based on a sound exposure level below that which causes a temporary threshold shift in the target and non-target species, for example an energy flux density of 120 dB re 1 μPa2s−1 above the hearing threshold of the targeted mammals.
According to a third aspect of the present invention there is provided an acoustic deterrent device comprising a signal transducer arranged to transmit acoustic signals in accordance with the method of the first aspect and/or the method of the second aspect.
According to a fourth aspect of the present invention there is provided an acoustic deterrent system comprising a control unit, a power source, amplifier and transducer means, co-operable to perform the method of the first aspect and/or the method of the second aspect.
According to a fifth aspect of the invention there is provided control software executable on a computer so that the computer is operable as the control unit of the fourth aspect.
The control software can be provided recorded on a computer readable medium, or made available for download.
According to a sixth aspect of the present invention there is provided an acoustic signal which incurs a startle reflex response in mammals in order to deter them. The acoustic signal can be produced and used in accordance with any of the previous mentioned aspects.
According to a seventh aspect of the invention there is provided an acoustic signal for the deterrence of mammals which comprises an aversive sound whose characteristics are chosen based on characteristics that are unpleasant to humans. The acoustic signal can be produced and used in accordance with any of the previous mentioned aspects.